/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        //判断环形链表，用快慢指针
        ListNode *p1 = head;
        ListNode *p2 = head;
        while (p1 != NULL && p1->next != NULL)
        {
            p2 = p2->next;
            p1 = p1->next->next;
            if (p1 == p2)
            {
                return true;
            }

        }
        return false;
    }
};